The present invention relates to a process of preparation of graft copolymers of styrene and an unsaturated ethylene-propylene-diene terpolymer.
Polystyrene is a thermoplastic material, which, owing to the ease with which it can be obtained and processed, is widely used in the plastics industry. It is also well known, however, that the use of polystyrene is limited applicationwise because of its poor heat stability and weak impact resistance. To improve these properties, it has been proposed to use copolymers of styrene with other monomers, such as acrylonitrile or acrylates. It has also been proposed to use graft copolymers, obtained by polymerizing styrene on a rubbery polymer. Proposed rubbery polymers included butadiene homopolymers and copolymers. However, in view of the fact that these unsaturated rubbers are very sensitive to oxidation and to the action of light, the resultant products have substantially less than ideal physical and mechanical properties.
To improve the styrene-rubber graft copolymers, it has been proposed to substitute for the unsaturated rubbers largely saturated rubbers, for example, rubbers of ethylene and propylene (EP rubbers) and rubbers of ethylene, propylene and of a polyunsaturated monomer (EPDM rubbers). However, their use presents numerous difficulties: in particular, these slightly unsaturated rubbers, having a low content of double bonds, do not facilitate the grafting of styrene on to the elastomer. Consequently, compositions are obtained which possess a very low degree of grafting, i.e., a very low degree of polystyrene grafted on to the elastomer, at most around 2. Besides, microscopic observation shows that the resultant compositions contain large rubber globules (up to 15 microns), which are dispersed in the composition without any cohesion. Thus, the resultant composition with this type of dispersion does not have a fine microstructure, which in turn results in products lacking good optical properties.
Apart from the problems described above, which relate to stability to heat and aging, impact resistance, the degree of grafting of polystyrene as well as microstructure, it is also necessary for obtaining compositions possessing useful properties, to solve a bad problem involving the crosslinking of the elastomer. ("Elastomer" and "rubber" are used synonymously herein.) When styrene is polymerized in the presence of EPDM elastomers, crude graft products are obtained, comprising, in particular, a graft copolymer, formed by the reaction of the elastomer with the styrene monomer and elastomer groupings, made up of "bridged" groupings, i.e. rubber groupings, linked to one another by chemical bonds. The quantity of "bridged" rubber is given by the degree of crosslinking, which is expressed by the "swelling index", which is inversely proportional to the degree of crosslinking, which means that the lower the swelling index, the higher the degree of crosslinking.
To attempt to control crosslinking, several processes have been proposed involving complex reaction steps: use of a mixture of special solvents, partial oxidation of the rubber before grafting, multiple step polymerization with steps involving different polymerization techniques including mass and suspension polymerization. Unfortunately, all of these processes are unsatisfactory and, in particular, lead to graft polymers which often still possess a low degree of grafting as well as an undesired degree of crosslinking and a coarse microstructure. For reference to the patent and polymer literature regarding these processes, attention is invited to French Pat. No. 2 320 950, French Pat. No. 2 263 260 and French Pat. No. 2 164 832.